3x20 - Dragon Ball Z Kai

Goku uses his Instant Transmission ability to take Gohan, Vegeta, and Trunks with him to the Hyperbolic Time Chamber inside Kami's Lookout. The chamber only has a capacity of two so Goku allows Vegeta and Trunks to go in first. Goku explains a years worth of training in the chamber is actually only one day in the outside world. When Trunks and Vegeta step inside the chamber, all they see is a vast amount of nothingness. While this is going on, the androids arrive at the Kame House and ask for Goku's whereabouts. Piccolo refuses to tell them, which results in a fight between Piccolo and Android #17. Piccolo overpowers #17 and tries to finish him off, but #17 manages to dodge the attack. Back at Kami's Lookout, Vegeta and Trunks are due to be coming out of the time chamber any second now, which will reveal if it is possible to go beyond the level of a Super Saiyan.