Ep 7. Ikkakumon's Harpoon Torpedo

Tai and Matt argue whether they should climb Infinity Mountain. Joe and Gomamon decide to climb the mountain by themselves. While on the mountain, they are attacked by a Unimon. Tai and Sora come to the rescue, and Gomamon digivolves into Ikkakumon and destroys the Black Gear that was controlling Unimon.