NCIS: Los Angeles is getting a visit from a special guest during "Three Hearts."

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Billed as a game-changing episode, "Three Hearts" will see the appearance of Hawaii Five-0 actor Daniel Henney.

Despite writer's David Kalstein ("Tuhon," "The Frozen Lake") and Kyle Harimoto ("Merry Evasion," "Omni") attempts to keep Henney's upcoming role under wraps, word slipped out and Henney was quick to confirm his involvement.

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"Secret's out:). NCIS LA ...Stay tuned! #threehearts #angelo #NCISLA," Henney tweeted on the last day of filming for episode 21.

Whatever Henney's role turns out to be, Kalstein has promised fans that it will be worth the wait.

"NCISLA "Three Hearts" Day 3: Wait till you guys see our special guest star's tour de force performance. Living the dream w/ [Harimoto]," Kalstein tweeted.

Meanwhile, Callen (Chris O'Donnell) and his crew will be busy chasing a case from the beach to an empty warehouse to Benedict Canyon while dealing with a girl, a knife, and a whistle (see photos above). As Sam (LL Cool J), Deeks (Eric Christian Olsen), and Kensi (Daniela Ruah) help Callen deal with matters out in the field, Hetty (Linda Hunt) and Granger (Miguel Ferrer) will be double-teaming a criminal back at the boat shed.

Scroll through the gallery above to see what the cast was up to while filming for "Three Hearts."

NCIS: LA airs Tuesdays at 9 p.m. ET on CBS. Click the video below to see a preview for the series' next episode, "Zero Days."