Opposites Interact! (Japanese: マンムー、パチリス！決めろ氷のシャンデリア！！Mammoo, Pachirisu! Decide on the Ice Chandelier!!) is the 175th episode of the Diamond & Pearl series, and the 641st episode of the Pokémon anime. It first aired in Japan on May 6, 2010 and in the United States on September 25, 2010.
Ursula is her usual assured self, and as the battle begins, it appears that Dawn, competing with her Mamoswine and Pachirisu, can't even get a move in. Ursula and her Gabite and Flareon seem unstoppable, as they chip away at Dawn's points little by little.
But Ursula's overconfidence provides an opening for Dawn. Inspired, she decides to use Ice Chandelier—her newest move—and pulls it off beautifully! When a surprised Ursula is caught off guard, Dawn uses Gabite's Dig to her own advantage...and when the smoke clears, Gabite and Flareon are unable to battle, giving Dawn a last-minute win!
Ursula, of course, chalks up Dawn's success to dumb luck. But no matter—Dawn has earned a place in the next round! Jessilina, Zoey, and Nando have won their rounds as well, and with that group of competitors, the excitement is sure to keep going!
Ash and Dawn begin training with their Pokémon, with Dawn practicing her Contest moves while Ash has Gible practices its Draco Meteor attack, only for it to head toward Piplup. Togekiss defends Piplup, slapping the attack away, only for the meteors to turn around and crash land with a direct hit into Ash and Gible. Once the match-ups for the Battle Stage are chosen, Dawn discovers that she will be facing Ursula next.
Dawn sends out Pachirisu and Mamoswine while Ursula chooses her Gabite and Flareon. Dawn orders Pachirisu to start off with Discharge, sending a surge of electric current at Flareon. Ursula counters easily by commanding her Gabite to block the attack, causing the Electric-type move to have no effect at all. However, this is what Dawn thought would happen. Pachirisu continues using Discharge, and it's shown that the move is also being aimed at Mamoswine. Dawn commands Mamoswine to attack Gabite with Take Down, and still surrounded by electricity, Mamoswine rushes at Gabite. The attack sends Gabite flying backwards, hitting Flareon and allowing the Discharge to injure it, causing Ursula to lose a chunk of her points. Gabite and Flareon pick themselves up and with a determined look, Ursula tells her Gabite to use Sandstorm. Gabite creates a furious cyclone of sand which Flareon avoids by jumping, but on Dawn's side the gales push at her Pokémon, blowing away Pachirisu. To counter, Dawn commands Mamoswine to use Ancient Power to part the Sandstorm. Mamoswine begins to charge the attack, but Ursula will not allow it to interfere. She has Flareon use Scary Face in combination with Sandstorm, creating a frightening image of a large, glowing-eyed Flareon in the whirling sand. Mamoswine is intimidated by the image and the attack fails, causing Dawn to lose a quarter of her points.
Dawn encourages her Pokémon to stay strong, and they pick themselves up. She orders Pachirisu to use Super Fang on Flareon and Mamoswine to use Ice Shard on Gabite. Pachirisu begins sprinting forward and Mamoswine starts charging its attack. Ursula commands her Gabite to evade the attack with Double Team, and the illusions not only confuse Mamoswine, but they also surround Pachirisu. She orders Flareon to attack with Shadow Ball, and the dark spheres pass through the illusions, hitting Dawn's unsuspecting Pokémon. This causes Dawn to lose another quarter of her points. Dawn calls out to her Pokémon again, telling them to do their best, and they get up. Ursula taunts Dawn and orders another barrage of Shadow Balls from Flareon. Pachirisu jumps around, avoiding the attacks, while Mamoswine takes them head on, but they eventually fall to the ground in fatigue. Gabite stops its Double Team in confidence, letting Mamoswine begin charging an Ice Shard. However, Flareon easily dissipates it with a Fire Spin. Ursula commands Gabite to use Dragon Claw, and the dragon swipes at Pachirisu, knocking another chunk of Dawn's points away in the process. Dawn tells Mamoswine to use Ice Shard again, but Ursula only commands another Fire Spin to stop it. When Fire Spin hits, the attack breaks into smaller pieces of sparkling ice. Despite this, Dawn orders yet another Ice Shard. Ursula tells Gabite to use Dragon Claw and Flareon to use Fire Spin, but Dawn counters with Pachirisu's Spark. The electric rodent flies past Gabite, sending Flareon backwards. With the Fire type out of the way, Dawn tells Mamoswine to release its Ice Shard, and the pieces fly toward Pachirisu, encasing it in glittering ice. Dawn finally orders a Discharge from Pachirisu, and the rodent charges forward in a shining ball of ice, successfully completing Dawn's new combo, Ice Chandelier. As Ursula stares in shock, over a quarter of her points deplete.
Ursula recovers quickly and orders Gabite to use Dig. Gabite burrows underground and Flareon hops into the hole after it. With no target, Pachirisu crashes it the wall, breaking the Ice Chandelier. Ursula's Pokémon jump back up after, and Flareon shoots a Shadow Ball at Pachirisu. Dawn commands Mamoswine to use Hidden Power, but Ursula's Pokémon repeat their Dig and Shadow Ball combination many times, leaving Dawn's remaining points slim. Ursula then tells her Gabite to use Dragon Claw. Dawn suddenly orders Pachirisu to jump into one of the many holes left behind and tells Mamoswine to fire another Ice Shard. Gabite collides into Mamoswine while Ursula orders Flareon to use Fire Fang. Mamoswine endures the attacks and sends its Ice Shard attack into the holes, and suddenly, a Discharge is fired out from the holes. Dawn orders Mamoswine to use Ancient Power, creating a fissure in the ground that reveals Pachirisu encased in another Ice Chandelier. Pachirisu manages to knock into both its opponents and turns around for another shot, but Ursula orders a Sandstorm and Fire Spin. In turn, Dawn commands Mamoswine to use Hidden Power on Pachirisu, causing the ice ball to accelerate exponentially. Before Gabite and Flareon can attack, Pachirisu crashes into them in a cloud of smoke and ice. Dawn begins to order another attack, but when the smoke clears, it's revealed that Gabite and Flareon are unable to battle, giving Dawn the win.
When Zoey and Nando are walking up to greet Dawn in the morning, the underside of Nando's hat turns a darker green for one frame.
While Marian makes her announcement at the beginning of the episode, the feather in Nando's hat is missing.
Marian comments "yesterday was the first round and today the second round begins". Instead, she should have said "two days ago was the first round". In the previous episode, Marian said there would be a rest day between the first and second rounds. Since Dawn and Ash were training their Pokémon outdoors in the beginning of the episode, it may have been implied this was the rest day Marian was referring to, and Dawn's battle against Ursula began the day after her outdoor training.
One of the Coordinator trading cards James and Meowth were selling depicts Ursula with her ribbons colored the same as her hair when they should have been lavender.
In one scene, the outline separating Ursula's arm and torso is missing.