If a spell played from the trash with Flow is countered, does it go to the trash or get banished?

Ruling: The spell is banished. Flow's banishment is a delayed replacement effect that applies whenever the spell would leave the chain after becoming a finalized chain item, unless leaving the chain was instructed by its own execution. A countered spell still leaves the chain as a finalized chain item, so Flow's replacement effect applies and banishes it instead of sending it to the trash.

Nuances:

• This prevents the spell from being playable again with Flow after being countered.
• Flow's replacement effect is broader than typical post-resolution replacements because it applies to any departure from the chain (except self-instructed departures), including counters.