Ben 10/Generator Rex: Heroes United (watch between S2E10 and S2E11 for continuity).
This season follows Ben's team—Gwen Tennyson and Kevin Levin—as they try to save five kidnapped aliens: Bivalvan, Galapagus, P'Andor, Andreas, and Ra'ad. ben 10: ultimate alien all episodes
Recommended episode structure (key episodes to watch; spoilers minimized) Ben 10/Generator Rex: Heroes United (watch between S2E10
The final season wraps up every loose thread, culminating in a massive battle between Ben, Vilgax (who has fused with Dagon), and an unexpected savior. Vilgax (who has fused with Dagon)