So what does it involve and what does it mean for all of us?
First, the basics
Since 1999 the decision-making on much that affects us here in Wales, such as health, education, agriculture and transport, has been devolved to politicians in Cardiff Bay.
Just as in the UK Parliament at Westminster, the largest party in the Senedd forms a government.
For all of that time the Welsh Government has been led by the Labour Party although it has never had an outright majority which has meant it has often had to reach deals with other parties to get its plans through.
Sometimes this has been a formal coalition government, first with the Liberal Democrats, then with Plaid Cymru, finally with two individual members, Kirsty Williams and Dafydd Elis Thomas.
At other times there have been other kind of arrangements, sometimes known as “supply and confidence” deals whereby one party wins concessions in return for backing the government in budget and confidence votes.
Labour and Plaid Cymru’s agreement is none of these!
Why are they doing it?
Following the Senedd election in May 2021, Labour won exactly half the seats in the Welsh Parliament, more than enough to govern but not enough to do so without the risk of the other parties ganging up to block its plans.
So, in December 2021, Mark Drakeford and Plaid Cymru’s leader Adam Price signed an agreement to work together.
Their Co-operation Agreement lasts for three years and means they will work together on 46 policy areas.
Both parties think this “bespoke” agreement gives them the best of both worlds.
Labour will avoid defeat on budget votes, confidence votes and, it hopes, on areas it sees as crucial to its programme of government.
Plaid Cymru thinks it will avoid the risk which going into a coalition government can create for smaller parties, can claim credit for tangible changes and can act as an opposition party in areas which it doesn’t agree with Labour.
What have they pledged to work on together?
Extending free school meals to all primary school children
Expanding free childcare to all two-year-olds
Committing to tackle the second homes crisis
Creation of a national care service
Long-term reform of the Senedd
Council tax reform
Investing more in flood defence and prevention
You can read the full agreement and see all the areas covered here.
Who’s in charge?
With no Plaid Cymru ministers, how will the rest of us know who’s been making the decisions?
It’s important to say that decisions are still Welsh Government decisions, whether or not they’ve been made by Labour ministers alone or in co-operation with Plaid Cymru.
For instance this week the Climate Change minister Julie James announced substantial investment in flood defences. Most of that was a Labour decision; she fronted the policy launch and answered questions about it.
But Plaid Cymru’s “designated member” Sian Gwenllian has also been speaking about the launch and explaining her party’s role in it.
She and Cefin Campbell are her party’s “Designated members” who will speak about policies covered in the agreement.
Will it work?
That’s a question yet to be answered.
When he signed the agreement, Mark Drakeford said: “This is a bespoke agreement to deliver for Wales but it also captures how Welsh politics works – by finding common ground and sharing good ideas.”
Adam Price said: “This pioneering Co-operation Agreement is founded on common ground on a range of issues that will make a long-lasting difference to people’s lives.”
But the Conservatives have criticised the deal as a “travesty” that voters “will rumble” quickly.
The Conservative Welsh Secretary Simon Hart said: “I mean, forget the legalities of it, seems to me to be very curious when you can be both government and opposition at the same time in the same building, and I'm amazed.
“I don't think people will tolerate that. I think people will rumble that quite quickly.”