Six chapters decide everything later. The rest of an agency contract is standard text that rarely comes up.
1. The scope
What gets done, expressed in work rather than hours: how many campaigns, how many adaptations, how many meetings, how many feedback rounds. And beside it, a list of what is not included. That second list is the most useful part of the whole contract, because that is where every discussion about additional work starts.
2. Who works on it
Names, or at least roles with a minimum seniority, plus an agreement on replacement: who decides, with what notice, and whether you may refuse a replacement. You chose on people, so you may write them down.
3. How you order more
The procedure for additional work, in two sentences: every extension gets a written estimate before work starts, and without approval nothing happens. This chapter prevents almost every invoice argument. See how to keep costs from running away.
4. Who holds the rights
To what is made, to the source files, and to work that was paid for but not used. By default copyright stays with the maker; what you need must be written down. Do not forget music, photography and fonts: those often carry licences limited in time and territory. See who owns the work when you stop.
5. How you terminate
A reciprocal notice period, what still happens during it, and how running projects are completed. See what a normal notice period is.
6. How handover works
Which files, access rights and accounts you get back, in what format and within what period. At the start this clause costs nothing. Afterwards it costs weeks. See how to make sure the handover goes well.
Two things that are often missing
Confidentiality in both directions, including what happens to your data when the relationship ends.
An exclusivity clause, or its explicit absence. May the agency serve your direct competitor? Both answers are defensible; no answer is not.
This is a general list and not legal advice; have the contract reviewed by someone qualified. IKAg sits on the advertiser's side of that negotiation, scope and remuneration included: see Agency evaluation & remuneration.
See also: how long an agency contract should run.