Can you tell me what is wrong with our plant?

Welcome to Garden.org, @Dannnnn !

The whole plant looks kind of pale to me, maybe needs some fertilizer? And the lower leaves look like they might be showing signs of blight or "wilt." Where are you located? (you can add that info to your profile and it will show up on the right-hand side of all your posts, which will be helpful with any future questions )

My plants started doing that last year in August when it was really warm and muggy. It started after rain and me stupidly using an overhead sprinkler to water them. I slowed it by cutting off every leaf that showed any signs of it and pruning to get more air in between the leaves. I didn't cure it though. Once the season was done I ripped up the whole thing, not letting ANY leaves stay behind, and burned it.

Not sure if that is the same thing or not, but those black spots sure look like it. I agree that the whole plant is kinda pale, that might be a different, additional issue.

Including pictures of what mine were doing.

How deep is the pot your plant is in? A tomato plant in a pot needs lots of depth for its roots, as well as lots of nutrients and water.

It's a bad idea to have the marigolds growing in there with it, too. They're robbing water and nutrients from the tomato plant.

It does look very unhappy.

If it is a nutrient problem, one symptom is "chlorosis between the veins". Many leaves are generally pale, except the veins seem to stay green longer.

BUT I forget what that means in terms of nutrients. Sorry!

Also I couldn't tell whether the yellowing was mostly the lower and older leaves, mostly young new top growth, or "generally some yellow everywhere".

Lack of mobile nutrients like N shows up first in older, lower leaves. The plant steals nitrogen from old parts to keep the new parts growing.

Lack of an IMMOBILE nutrient shows up first in new leaves and new growth becoming pale, lime-green, or yellow. The plants can't get enough, and can't move it around internally, so new growth has to do without.

It might have multiple problems that I can't guess -
- something eating the roots,
- something eating the leaves and injecting a plant disease
- other disease
- severe nutrient imbalance (bad pH or some excess nutrient making other nutrients unavailable)
- poor drainage rotting roots
- ???

I am a little late to the rodeo but this may help you or someone else. I have had what looks like the same issue with my tomato plants for the last few years. It usually starts when the weather really heats up and it works it's way through all my tomato plants. I usually use straw bales as a growing media. This year, because of a recent move I am growing my vegetables in 5 gallon buckets. About a week or two after transplanting my Celebrity tomatoes I noticed this stuff popping up on the leaves. I am not sure but I believe it is early blight or Septoria leaf spot. Both are a type of fungus.. Any way I contacted my local extension agent and she recommended I cut off all the effected leaves and start spraying them with Daconil. I took her suggestion and I I spray all the leaves top and bottom with the Daconil once a week. It seems to have pretty much stopped the spread of it. Daconil will not kill it but it can arrest the spread of it. It has been close to a month since I started spraying. I caught it early but I still had to cut a lot of leaves and entire branches off of the plants. It slowed them down a bit but they are bouncing back. I still find an occasional spot on a leaf or two but I immediately cut it off. Another thing I did was start a batch of Better Bush tomatoes just in case. I sprayed them with Daconil immediately after transplanting. That has been around two weeks ago, So far no sign of the blight. I am also growing bell peppers, cucumbers and watermelons. I have been spraying them with it also just to be safe. Cucumbers and melons have the tendency to get fungal infections here. The peppers were in the same enclosure as the Celebrity tomatoes but it seems the Daconil has helped keep the blight from spreading to them. From now on I plan on spraying my plants with an anti fungal right off the bat. The blight/fungus is easier to prevent than it is to control. I hope this helps. Good luck and take care. To add: There are also several organic DIY recipes for anti fungal sprays.