sed: joining lines depending on the second one

This might work for you:

sed 'N;s/\n\s*+//;P;D' file

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These are actually four commands:

• N
  Append line from the input file to the pattern space
• s/\n\s*+//
  Remove newline, following whitespace and the plus
• P
  print line from the pattern space until the first newline
• D
  delete line from the pattern space until the first newline, e.g. the part which was just printed

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The relevant manual page parts are

• Selecting lines by numbers
• Addresses overview
• Multiline techniques - using D,G,H,N,P to process multiple lines

Doing this in sed is certainly a good exercise, but it's pretty trivial in perl:

perl -0777 -pe 's/\n\s*\+//g' input