Okay, trying yet another summary....
The FPL is generally used as a basis to initiate a SAR op when a plane is overdue. For this purpose, a flight plan needs to be submitted and opened/activated. The FPL is then transmitted via the AFTN to the arrival A/D, who will use the airborne time and the EET to calculate the arrival time. If a plane is more than 30 minutes overdue at this arrival time they will initiate SAR action. Obviously, they will first ring a few likely places where the plane might have landed, and try to contact both the owner and the pilot, before the rescue choppers actually get into the air. To prevent SAR action, it is very important to close your flightplan, and to pass on any changes to the plan, for instance a revised ETA or diverting to an alternate.
Submitting a flight plan: Flight plans are submitted to ATC units that are ready to accept them. This can be done by internet, fax and sometimes by telephone as well. Consult your countries AIP to see how and where you can submit a flight plan. Typically, your local airfield staff (be it uncontrolled or controlled) will be able to help you out. Changing a flight plan before flight, or cancelling a flight plan before the flight takes place is handled in the same way.
Opening (activating) a flight plan: Flight plans are opened by the tower on a controlled airfield. As pilot, you don't need to worry about this. On uncontrolled fields, the responsibility to open a flight plan lies with the pilot. There are two ways of getting the flight plan opened in this case: You can nominate a "Responsible Person" at your departure airfield to phone your departure to a "Parent ATSU" (this is only done in the UK), you can ask the information or A/G service on the airfield to phone the coordinating ATC unit, or you can ask the first en-route ATC unit that you talk to to open the plan for you. In both cases, you are required to pass on the Actual Time of Departure (ATD).
Chaninging a flight plan en-route is generally handled by the en-route ATC unit you're talking to.
Closing a flight plan is automatically and implicitly done when you arrive at your destination, provided that your destination is a controlled airfield. If your destination is an uncontrolled field however, there are several options, and the situation in the UK is different from the situation elsewhere in Europe:
In continental Europe and in Ireland, it is the pilots responsibility to close the flight plan explicitly. This can be done while still in the air by contacting an en-route ATC unit when you are close to your destination and ask them to close the plan, or by phoning a Flight Information Office within 30 minutes after landing. Your countries AIP will have the details on who to call, and the destination airfield might have an information or A/G service which can help you out as well.
In the UK, if you fly to an uncontrolled airfield, the flightplan is also not formally closed automagically. In fact, the UK doesn't have a concept of "closing a flightplan". Because of this, if you fly to an uncontrolled airfield, your flight plan is not used as a basis to initiate a SAR action. Instead, the UK uses the concept of Responsible Person again. This RP is supposed to receive both a departure and an arrival message, and is supposed to initiate a SAR action if the plane is overdue. You can nominate your friend Bob as your RP, in which case you give him a call before and after the flight. But you can also ask (by phone, before the flight) an ATC unit (typically a "Parent ATCO") as your RP for the flight. In this case you need to give them a phonecall within 30 minutes after landing to notify them of your safe arrival (and thank them for their services). Generally you do not need to inform a Parent ATCO of your departure, since they will automatically receive a DEP message through the AFTN - provided that you picked the right Parent ATCO of course (the one which has Parent ATCO responsibility for your destination).
Note - I have assumed here that all controlled airfields are somehow connected to the AFTN, and all uncontrolled airfields are not. That's not necessarily true however, but as a pilot, it's going to be very hard for me to find out whether a field is properly connected or not. But I do know whether a field is controlled or not. If in doubt, call your destination beforehand!
Better???