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ParametricPlot3D[{Cos[u] (3 + Cos[u/2] Sin[v] - Sin[u/2] Sin[2 v]), 
  Sin[u] (3 + Cos[u/2] Sin[v] - Sin[u/2] Sin[2 v]), 
  Sin[u/2] Sin[v] + Cos[u/2] Sin[2 v]}, {u, 0, 2 Pi}, {v, 0, 2 Pi}]

How to additionally draw z-level curves (by Show or other means) in the above plot of Klein Bottle ( from documents)? Truncating $z$ by a PlotRange setting shows only a single pair of intersections on the surface per plot.

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Use "MeshFunctions", as in:

ParametricPlot3D[{Cos[u] (3 + Cos[u/2] Sin[v] - Sin[u/2] Sin[2 v]), 
Sin[u] (3 + Cos[u/2] Sin[v] - Sin[u/2] Sin[2 v]), 
Sin[u/2] Sin[v] + Cos[u/2] Sin[2 v]}, {u, 0, 2 Pi}, {v, 0, 2 Pi}, 
MeshFunctions -> {#3 &}, Mesh -> {{-1, 0, 1}}, MeshStyle -> Thick, 
PlotStyle -> Opacity[0.2], PlotPoints -> 100]
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An approach...take the expression for the Z terms, and solve them for a term Z

zterm = Sin[u/2] Sin[v] + Cos[u/2] Sin[2 v]
Solve[zterm == zz, {u}]

you get an expression that includes

{u -> ConditionalExpression[2 ArcTan[(
  zz Sin[2 v] - 
 Sqrt[-zz^2 Sin[v]^2 + Sin[v]^4 + Sin[v]^2 Sin[2 v]^2])/(
Sin[v]^2 + Sin[2 v]^2), 
Csc[v] (zz - (zz Sin[2 v]^2)/(Sin[v]^2 + Sin[2 v]^2) + (
 Sin[2 v] Sqrt[-Sin[v]^2 (zz^2 - Sin[v]^2 - Sin[2 v]^2)])/(
 Sin[v]^2 + Sin[2 v]^2))] + 2 \[Pi] C[1]), 
 C[1] \[Element] Integers]

Pick off the equation in the conditional expression and assign it to uu

uu = 2 ArcTan[(
  zz Sin[2 v] - 
   Sqrt[-zz^2 Sin[v]^2 + Sin[v]^4 + Sin[v]^2 Sin[2 v]^2])/(
  Sin[v]^2 + Sin[2 v]^2), 
  Csc[v] (zz - (zz Sin[2 v]^2)/(Sin[v]^2 + Sin[2 v]^2) + (
 Sin[2 v] Sqrt[-Sin[v]^2 (zz^2 - Sin[v]^2 - Sin[2 v]^2)])/(
 Sin[v]^2 + Sin[2 v]^2))]

Then do a 2D ParametricPlot, replacing u with uu. Here you can manipulate it to see what you have.

Manipulate[
   ParametricPlot[{Cos[u] (3 + Cos[u/2] Sin[v] - Sin[u/2] Sin[2 v]), 
   Sin[u] (3 + Cos[u/2] Sin[v] - Sin[u/2] Sin[2 v])} /. {u -> 
    uu} /. zz -> height, {v, 0, 2 Pi}], {height, -1, 1}]

enter image description here

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