This sounds like a textbook case of [Surprise](https://roll20.net/compendium/dnd5e/Combat#toc_2)

In general:
> 1. Determine Surprise. The GM determines whether anyone involved in the combat encounter is surprised.
2. Establish positions: The GM decides where all the characters and Monsters are located. Given the adventurers’ Marching Order or their stated positions in the room or other location, the GM figures out where the adversaries are̶how far away and in what direction.
3. Roll Initiative: Everyone involved in the combat encounter rolls Initiative, determining the order of combatants’ turns.
4. Take turns. Each participant in the battle takes a turn in Initiative order.
Begin the next round. When everyone involved in the combat has had a turn, the round ends.

Note that during Surprise,

>  Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.

> If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a Reaction until that turn ends.

In this case, the order of events is...

1. Combat positions are set up / drawn up / etc (or have already been done)

2. Everyone rolls initiative

3. Players each take a single turn.  There are a couple of ways to DM this - I couldn't find a RAW interpretation on *exactly* what order this occurs, since the  Wizard may not have the highest initiative but is taking the first action.

4. Combat enters the normal initiative, proceeds accordingly