Mystery Mask Mix-Up is the second episode of the second season of Scooby Doo Where Are You!, and the 19th episode overall in the series.
Daphne buys a decorative, and purportedly cursed, mask in Chinatown, and is subsequently kidnapped by a pair of ghastly ancient Chinese figures. The rest of the gang trail the kidnappers to the temple of the evil Zen Tuo, who is apparently haunting the temple and wants to do away with the gang.
In a seemingly abandoned temple in San Francisco's Chinatown, the Ghost of Zen Tuo materializes in a puff of smoke. He orders his minions to retrieve his mask, and not return without it.
Elsewhere in Chinatown, Fred Jones, Velma Dinkley, Daphne Blake, Shaggy Rogers and Scooby-Doo are watching the Chinese New Year parade. After the parade, the gang stops by at an antique store and Daphne purchases a golden Chinese mask encrusted with jewels. The gang leaves and two hooded ghosts arrive at the antique store and ask for the mask. The store owner, frightened, tells the ghosts that a girl already purchased it.
While walking down the streets, the gang runs into the hooded ghosts who attempt to take the mask. Scooby creates a smokescreen from a steam press and the gang flees. Mystery Inc. heads to an oriental art shop where a dealer, Mr. Fong, informs them that many centuries ago, the mask was stolen from the tomb of Zen Tuo, a Chinese warlord whose ghost returns to seek the mask. He warns them that they are in great danger.
While the gang is driving in the Mystery Machine, the hooded ghosts start following them in their car. A car chase ensues, leading the gang to crash into Fisherman's Wharf and land in a pile of fish. Afterward, Daphne is kidnapped (bound and gagged in the back seat of the ghosts' car) and the mask is taken and the hooded ghosts drive away. The kidnappers leave behind a paper that, when reflected in a mirror, reads "The Temple in the Hills".
The gang heads to the temple and enter. Splitting up, Fred and Velma find a secret room. They enter, hear a noise and look to see a bound and gagged Daphne. She struggles to speak through her gag. They remove it and she says it's a trap: The door closes and locks, and they are all trapped inside.
Scooby and Shaggy run into Zen Tuo (now wearing the golden mask) and the hooded ghosts who chase them. They eventually are captured and placed in a room tied up on a pile of fireworks, a lit fuse making its way toward them. They manage to escape, and in the process, let Daphne, Fred and Velma out of the room.
Daphne leads the gang to a secret room that she saw on her way in, full of pigeons in a coop. They find messages that read dates and times have been tied to the pigeons' legs.
Freddy formulates a trap, where Zen Tuo comes in, he steps on a train powered by Roman candles, and the trap is set. But Scooby doesn't finish laying the track pieces out and the train runs into Scooby. Despite this, the villain is captured.
Zen Tuo is Mr. Fong, and the hooded ghosts are his henchmen. Mr. Fong has been the leader of an international smuggling ring. Information about dates and times entered the country inside masks and then the info was sent to other gang members by the pigeons. When the police question how the gang got the mask in the first place, Daphne explains that it was mistakenly delivered to the antique shop where she herself bought the mask by sheer accident. The ghost act was to scare people away from the temple, their center of the operation.
With the case settled, Shaggy and Scooby are now concerned about their next meal; Velma offers a Scooby Snack, but she only has one for them both. She tosses it into the air for them to catch, but they fumble and fail to catch it, hitting the floor and being momentarily knocked out. In that moment, a mouse runs out from its hole and takes the Scooby Snack for itself; eliciting laughs from everyone about them being outfoxed by a mouse.
- Scooby can speak Chinese and Velma can read it (and therefore presumably speak it, too).
- H&B Tuna Co. is a reference to Hanna-Barbera.
- Whilst not yet retconned, Daphne's wealth is implied in this episode, as she is able to buy an ornate golden mask inlaid with jewels as an impulse purchase. On the other hand, having been delivered by mistake to a curio shop, it may not have been that expensive.
- Zen Tuo is the second villain to attempt to kill members of the gang, after the Snow Ghost in "That's Snow Ghost".