I have out a Vesuvan Doppelganger with three +1/+1 counters. During my upkeep I target my opponent's Triskelion with the Doppelganger's shapeshifting ability. His Triskelion has one +1/+1 counter. In response to me targeting the Triskelion with the Doppelganger's ability, my opponent removes the counter from Triskelion to kill the Triskelion.
Our question, since the target is no longer on the battlefield does the ability of the Doppelganger "fizzle"? And if so, does it just remain the creature it was at the start of the turn?
Once your upkeep begins, the triggered ability granted by Vesuvan Doppelganger will trigger, and then go on the stack, when you can target Triskelion; this happens before any player gets priority to activate abilities, such as Triskelion's second ability (C.R. 108.1, 117.5, 603.3d, 601.2c, 117.1b; C.R. 503.1, especially C.R. 503.1a).
The triggered ability granted by Vesuvan Doppelganger targets a "creature" according to its Oracle text (C.R. 108.1, 115.1d). Thus, if that creature becomes illegal, the ability will fail to resolve (C.R. 608.2b).
See also for example Cryptoplasm where a completely similar situation is actually described in the additional rulings in gatherer:
"If the creature is an illegal target when the ability tries to resolve, it won’t resolve. Cryptoplasm won’t become a copy of that creature; it remains whatever it was before."
As usual, the gatherer is rather incomplete when it comes to what cards are covered with additional rulings, and what cards are not, unfortunately.
Our question, since the target is no longer on the battlefield does the ability of the Doppelganger "fizzle"? And if so, does it just remain the creature it was at the start of the turn?
The triggered ability granted by Vesuvan Doppelganger targets a "creature" according to its Oracle text (C.R. 108.1, 115.1d). Thus, if that creature becomes illegal, the ability will fail to resolve (C.R. 608.2b).
EDIT (Jan. 5): Correctness edit.
"If the creature is an illegal target when the ability tries to resolve, it won’t resolve. Cryptoplasm won’t become a copy of that creature; it remains whatever it was before."
As usual, the gatherer is rather incomplete when it comes to what cards are covered with additional rulings, and what cards are not, unfortunately.
Cheers!
Zappa from Cairo
We did play it out correctly but were just a little unsure about it.