The heroes cycle to Hearthome City. Ash is prepared to challenge the Gym leader, but Brock reminds him the Gym is much more distant than the distance they can travel with their bikes. The heroes see a storm approaching and Brock's egg glowing. They hurry to go to the nearest Pokémon Center. However, the Center looks deserted and abandoned. They see a Chansey and tell her they need to have the egg hatched soon. They come to Nurse Joy and Brock begins to flirt with her, but they see something is troubling her.
The heroes introduce themselves, but Joy does not listen and tells them the Center is her fault. Since the Cycling Road has been built, the Center has been forgotten. Brock tries to cheer her up by working - the egg needs to be hatched soon. Joy tells him she is not good, but Brock continues and she takes the egg. The heroes come inside (as the rain chases them inside) and they prepare for the hatching. Joy is nervous, but with Chansey's patting, she gets control of herself. Brock tells Ash and Dawn many towels and water need to be for the hatching.
Team Rocket are happy to have a roof and food - they are in the Pokémon Center. They hide just in second before Dawn comes to check if there are any towels. Ash and Dawn got the materials and they see the hatching. The egg hatches into a small Pokémon named Happiny. Everyone is amazed and Brock gives her a bath. Brock introduces Happiny to everyone and thanks Joy for the assistance. Team Rocket recall Happiny evolves into a Chansey. Jessie sees her medal and recalls her days at the Nurse school. Jessie already plans to get Happiny, evolve it into Chansey and build a hospital of her own to become a nurse she should've been. They sneak away and prepare for what is to come.
Happiny gets her first meal. She begins to cry, as she wants a stone to imitate Chansey, so Brock goes to get one. Happiny cries even more, so Ash and Dawn go to get a stone. After some time, they return with many stones and Brock picks one of them. He polishes and brushes it to be perfect. Pikachu plays with Happiny and Brock polished the stone and goes to give it to Happiny, but Team Rocket steal her. Team Rocket tell Joy they will start their own Center - for profit. Pikachu goes to use Thunderbolt, but Jessie brings Dustox, who stops them with Poison Sting.
The heroes still go after the balloon, but get stopped by Cacnea's Sandstorm. Jessie plays with Happiny and tosses her, but she gets on the balloon. Wobbuffet and Jessie go to get her down, but the weight rips the balloon and they fall down. Ash sends Staravia, who uses Whirlwind to blast the sandstorm away, then goes to search for Team Rocket. Joy blames herself for all this trouble, even if Ash told her Team Rocket took her. They see a Starly and a Weedle, which must've been hurt by Sandstorm. Brock tells Joy she needs to heal them and she accepts the task.
Jessie and Meowth try to entertain Happiny, but fail, as James tells them she needs a stone to hold. Staravia spotted them and goes back. Joy treated Starly and Weedle and Brock proves her she has some Pokémon to heal from the forest. Staravia came back and reports Team Rocket's location. Jessie picks the stones, but Happiny does not like any of them. Jessie is angry and goes to get her, but James and Meowth stop her. The heroes arrive and James sends Cacnea (and dodges it). Cacnea uses Pin Missile, but gets hurt by Chansey's Egg Bomb. Jessie sends Seviper to bite, but Brock sends Sudowoodo that counters its move using Double Edge. Cacnea uses Needle Arm, hitting Sudowoodo and throwing him back to Brock.
Happiny goes to get to him, but gets stopped by Meowth, Wobbuffet and Mime Jr. Seviper goes to use Poison Tail, but Happiny stops it (as she came through Meowth, Mime Jr. and Wobbuffet), and throws it back to Team Rocket. Cacnea uses Pin Missile and Seviper Bite, but get blasted off by Pikachu's Volt Tackle. Brock gives Happiny the stone and she likes it. Brock plays with her, but this ends in Happiny playing with him instead. Brock takes his Poké Ball out and catches Happiny.
The heroes get the Pokémon Center in order and Joy reports the Pokémon are all fine. Joy tells them the Hearthome Gym is one of the nicest Gyms in Sinnoh. She is grateful for such assistance and will treat the Forest Pokémon. Brock goes to flirt with her, but gets jabbed by Croagunk. The heroes go back on the road to Hearthome City.
- Brock's Happiny (newly hatched)
- The title is based on the saying "One big happy family".
- Ash finally finds out that Pikachu destroyed Dawn's bike.
- Jessie remembers her old friend Blissey, who last appeared in Ignorance is Blissey.
- Music from Pikachu's Vacation and Pikachu's Rescue Adventure is used as background music.
- Also, music from Pikachu's Pikaboo and Gotta Dance! is used in this episode.
- Professor Oak's Pokémon lecture: Bidoof
- Pokémon senryū: パッとでた ビッパのでっぱ りっぱかな Patto deta, Bippa no deppa, rippa kana "I gave it a pat, Bippa's overbite, it's splendid, I wonder?"
- Professor Oak narrates the preview for the next episode
- This is the first episode in which Emily Jenness voices Nurse Joy's Chansey.
- 'Explosive Birth' (爆誕) is also part of the title of the second Pokémon movie in Japan, The Power of One.
- When Staravia gets rid of the Sandstorm, Ash tells it to use Whirlwind in the english dub instead of Gust.