When Junior is kidnapped as leverage to retrieve the stolen list of undercover CIA agents, and most of Five-0 is rounded up by the CIA to prevent them from interfering, Tani and Quinn ask Magnum and Higgins for their help.
The crossover starts on Hawaii Five-0 S10E12 Ihea ‘oe i ka wa a ka ua e loku ana? (Where were you when the rain was pouring?) (I).
